The conventional combustion engine of this kind is known to be so designed that, prior to scavenging of a combustion chamber with an air-fuel mixture, the combustion chamber is initially scavenged with an air to suppress the blow-off of the air-fuel mixture through an exhaust port. (See, for example, the Japanese Laid-open Patent Publication Nos. 2001-173447 and 58-5424.)
It has been found that while in the two-cycle combustion engine of this air scavenging type, bearings disposed in the engine cylinder block for the support of the crankshaft are lubricated with the air-fuel mixture introduced into the crank chamber, an attempt to make the combustion engine of this kind compact tends to reduce the size of a gap through which the air-fuel mixture within the crank chamber to such an extent as to make it difficult to lubricate the bearings. For this reason, formation of oil supply passages to lubricate the bearings effectively renders the engine structure complicated.